1. Field of the Invention
The present invention relates generally to a charged beam lithography system. More specifically, the invention relates to a charged beam lithography system capable of rapidly and precisely correcting the proximity effect caused under the influence of backward scattered electrons produced by the irradiation with charged beams, such as an electron beam or an ion beam.
2. Description of the Prior Art
With the higher density integration of semiconductor devices, a charged beam lithography system for writing a pattern on a substrate, such as a wafer or a reticle, using charged beams, such as an electron beam or an ion beam has been developed.
Since a typical semiconductor device is designed by means of a computer aided design (CAD), writing data for a charged beam lithography system are also prepared using CAD data.
However, since the writing data have a characteristic data format, there is a problem in that it takes very much time to convert the CAD data.
In addition, in the pattern lithography using charged beams, there is a problem in that the charged beam exposures of adjacent patterns is increased by scattered electrons (backward scattered electrons, forward scattered electrons) produced from a substrate irradiated with charged beams such as EB(Electron Beam). For example, as shown in the schematic diagram of FIG. 1, if a certain point P is irradiated with an electron beam 311 in order to write a pattern on a substrate 310, scattered electrons (backward scattered electrons) are produced from the substrate 310 by irradiation energy supplied onto the point P. Since the amount of backward scattered electrons thus produced is maximum at the point P and decreases with the increase of the distance from the point P, energy 312 of the backward scattered electrons is distributed as if contour lines of concentric circles about the point P are drawn. For example, in the case of an electron beam lithography system having an acceleration voltage of 50 kV, electrons scattered backward have an influence on a range of about 10 to 20 .mu.m (back-scattering range .sigma..sub.b) from the incident point P. Therefore, even if the same dosage is applied, the actual EB exposure to adjacent patterns is different from that to isolated patterns. This is a phenomenon called the proximity effect. Under the influence of the proximity effect, the shape of a desired pattern deteriorates, and the dimension thereof varies. Therefore, with the scale down of patterns, it is an important problem to correct the EB exposure to suppress the influence of the proximity effect. Conventionally, in order to solve this problem, an auxiliary region having a size based on a back scattering range .sigma..sub.b was provided around a pattern to be written and the optimum correcting dosage has been corrected by means of a data processing with a software or by means of a hardware in an electron beam lithography system taking account of the influence of the backward scattering from the auxiliary region.
However, if the proximity effect correction is carried out by means of a software when a pattern is written on a reticle, it takes a lot of time to make a calculation since there are mass data to carry out a correction processing. Therefore, in order to shorten the processing time required for the correction processing, a high-speed computer outside the electron beam lithography system is utilized, and a parallel processing is utilized, similar to a case where patterns are written directly on a wafer.
As one of methods for solving the above-described problem by means of hardware, there is a method for providing a real time proximity effect correcting circuit in an electron beam lithography system. As the form of a real time proximity effect correcting circuit, there is a first example of a method in which correction of data of all patterns on a reticle are batch-processed and the patterns thereon are written using the corrected data. However, since this method can not write patterns unless the correction in the whole area of the reticle is completed, the total throughput of the electron beam lithography system is small. There is also a second example of a method for dividing writing data into stripes to use each of the stripes as a writing unit to carry out the correction and writing every stripe by the pipeline processing. According to this method, the loss time of the electron beam lithography system is far less than that in the first example. Furthermore, in these examples, the optimum dosage data are set every region of about several .mu.m.
In a case where a plurality of patterns for semiconductor chips are written on the same substrate, there are some cases where the chips are arranged at shorter intervals than the backward scattering range .sigma..sub.b. Conventionally, since the proximity effect correction is carried out in each of the chips, the proximity effect has an influence on the patterns in the end portion of each of the chips, so that it is not possible to obtain a target precision in that portion. In order to avoid such a situation, a method for integrating (merging) a plurality of chips into one chip at the stage of the CAD data is considered. However, in order to cause the proximity effect correction to be reflected in the whole chip, the conversion of the CAD data into the writing data must be carried out again, so that it takes very much time to carry out the processing.
In addition, there are some cases in that patterns to be written include a pattern (which will be hereinafter referred to as a "protruding pattern") which is to be divided by the boundary line between adjacent two stripes formed by dividing the whole region to be written. There may be some cases in which, if a protruding part locates outside a certain stripe by a predetermined range, such protruding pattern is written together with the stripe. For example, in the above described first example, the calculation for correction is carried out on the whole surface, so that there is no problem since the inside and outside of the stripes are distinguished from each other. However, the performance of the whole system remains being low. On the other hand, in the second example for carrying out the calculation for correction every stripe, there is a problem. That is, since the auxiliary region for calculating the proximity effect correction is not defined with respect to a portion protruding from a certain stripe, it is not possible to precisely calculate the corrected dosage. For that reason, the corrected dosage is outputted with respect to only patterns contained in the same region as the stripe region, so that it is not possible to set the corrected dosage in view of the patterns in the protruding region.
For example, in patterns shown in FIG. 2, if the size of the above described stripe is defined by the maximum deflection widths of a main deflecting system and sub-deflecting system of the lithography system to divide the whole region to be written by the defined stripe, a pattern 151 arranged in a sub-field A is divided by a boundary line between a stripe 1 and a stripe 2. In a case where a corrected dosage is calculated every stripe, the corrected dosages on the regions of the pattern 151 belonging to the stripes 1 and 2 are separately calculated. As a result, since the influence of the proximity effect affected by patterns belonging to the adjacent stripes is not considered, there is a problem in that the dimensional precision of the pattern 151 deteriorates in the boundary region between the two stripes, so that it is not possible to write patterns as designed.
On the other hand, separately from the proximity effect correction, in recent years, a multiple writing technique is utilized as a technique for enhancing the dimensional precision and positional precision of patterns. There are various multiple writing methods, one of which is a stripe multiple writing for writing stripes while shifting each of the stripes by a predetermine distance. If this stripe multiple writing and the above described first example of the proximity effect correction are combined to be carried out, there is no problem in the multiple writing of stripes since the irradiation time on the whole reticle has been calculated. However, since it takes very much time to calculate the correction as described above, there is a problem in that the loss time remains being great.
If the stripe multiple writing is combined with the second example, there is the following problem since the proximity effect correction is processed every stripe. That is, assuming that the number of stripes is n (n: natural number) in the writing data stage, when the multiple writing is carried out four times, the correction corresponding to 4n stripes must be carried out. Therefore, it is required to improve the processing speed of the correcting circuit in accordance with the 4n stripes.
Thus, in the multiple writing, it is difficult to effectively incorporate a proximity effect correcting circuit. In addition, if the region of the dosage data is expanded so as to include the auxiliary region in order to carry out the proximity effect correction with respect to the above described protruding pattern, there is a problem in that the coordinates of the dosage data corresponding to the intermediate stripe for the multiple writing are shifted, so that it is not possible to carry out a precise setting.